Easiest charging method I've seen is the first one you mentioned. Hot gas, 110F + OAT = Discharge temp. This will put you "in the ball park"
Other people will say the only way to charge a heat pump is to reclaim and weigh it in according to manufacture specs, but my boss would tell me that takes too long.
The third port is your dedicated suction/low side port, it connects into the refrigeration circuit before the reversing valve.
Your high side hose goes to the discharge line (larger of the two lines in the lineset)
On a Trane HP, pull off the side panel and the ports are on the reversing valve. Discharge is on top by itself, suction is in the middle on the bottom.
Hope this helps you out a little.